Field of the Invention
The present invention relates to a process and a device for controlling the speed of an internal combustion engine during a deceleration phase, and more specifically to a process and a device of this type which work by correcting the control of an actuator which affects this speed as a function of the deviation between a set-point speed and the actual speed.
Internal combustion engines, particularly those which drive automobiles, run at variable speeds, the control and/or adjustment of which is often tricky, particularly during the deceleration phase. A deceleration phase usually begins when the driver lifts his foot off the accelerator. The purpose of speed control during such a phase is to ensure the return of this speed to a set-point speed, the adjustment of the speed to around this set-point speed in spite of potential disturbances, and the passage through various transitory phases such as a "driven" deceleration phase in which the vehicle runs with an engaged gear box ratio, or a startup phase of the engine.
In each of these circumstances, control of the speed is quite tricky, since it is known that the stability of an engine is difficult to ensure at low speed and that the performance of the engine is difficult to model. Moreover, the conditions for the onset of a deceleration phase can vary considerably, for example in terms of the driver's action on the accelerator pedal, the temperature of the engine coolant, the air temperature, and the potential presence of random disturbances due to the engagement of an electrical device (lighting device, ventilator) or mechanical device (air conditioner, power steering). The speed control must also take into account other constraints associated with the driver's comfort (noise level, vibrations, jerking) and to standards related to the pollution of the environment by the exhaust gases from the engine.
At present, in order to ensure the control of the speed of an engine during deceleration, closed loop control devices with "supervised" PID-type controllers are commonly used. A device of this type is described in German patent disclosure DE-A-4 215 959, for example, which uses fuzzy logic to adjust the P, I and D terms of the controller. The result is a time-consuming, tedious tuning of the controller to adapt it to each type of engine. The PID adjustment is also disadvantageous in that it only takes into account certain aspects of the operation of the engine, and in that it is not entirely satisfactory from the point of view of "robustness", since the aging of the engine or the manufacturing tolerances for engines can unfavorably affect the operation of a "supervised" PID controller.
Another process for controlling the deceleration speed of an internal combustion engine, which is based entirely on experimental results formalized with the aid of fuzzy logic and is therefore likely, a priori, to have greater robustness and flexibility, is known from document No. 900594 published by the Society of Automotive Engineers of the United States of America. However, the process described requires the utilization of tables and complex operators which take up a lot of space in the memory of the computer used to implement the process, which also means long calculation times.
The object of the present invention is to provide a process for controlling the speed of an internal combustion engine during deceleration which will be satisfactory from four points of view: robustness, resistance to disturbances, ease of adjustment and the pleasure of driving a vehicle propelled by such an engine, in all the phases of deceleration.
Another object of the present invention is to produce a device for implementing this process.
These objects of the invention, as well as others which will become apparent through a reading of the description which follows, are achieved by means of a process for controlling the deceleration speed N of an internal combustion engine by correcting the control of an actuator which affects this speed as a function of the error E=N.sub.c -N between a set-point speed N.sub.c and the actual speed N; this process is remarkable in that it establishes the locus of the ideal states of the engine, defined by the pairs of specific values of the error E and its time derivative E' which correspond to the states of the engine which allow the set-point speed N.sub.c to be regained without correcting the control of the actuator, by means of a monotonic, rapid and smooth variation of the speed N, and in that it corrects the control of the actuator as a function of the deviation between the actual state (E, E') of the engine and the locus of the ideal states of this engine.
As will be seen below, control of the deceleration speed of the engine is optimized and simplified by the establishment of the locus of the "ideal" states of the engine according to the invention.
In another characteristic of the process according to the invention, a correction value .DELTA.u for the control of the actuator is drawn from a table with two inputs constituted by the error E and the derivative E' of the error, respectively. The table contains specific values for the correction .DELTA.u of the control of the actuator, each of which is associated with a pair of specific values of the error E and the derivative E' of the error.
In an advantageous variant of the process according to the invention, the correction .DELTA.u of the control of the actuator is drawn from a linear combination of partial corrections taken from this table and from a second table, respectively, which second table corresponds specific values of the partial correction it determines with specific values of the derivative E' of the error. Thus, it is possible to adapt the control of the deceleration speed of the engine to various operating conditions of the engine simply by modifying the coefficients of the linear combination.
The present invention also provides a device for implementing this process, which includes a) means for outputting a first signal representing the speed error E and a second signal representing the derivative E' of this error, derived from a signal output by a sensor of the actual speed N of the engine and from a signal representing a predetermined value of the set-point deceleration speed N.sub.c and b) a controller supplied with these first and second signals which draws a correction value .DELTA.u for the control of the actuator from these first and second signals, and means for storing specific values of this correction .DELTA.u as a function of the deviation between the actual state (E, E') of the engine as known by means of these signals and the locus of the ideal states of this engine.